The Origin of High Efficiency in Low-temperature Solution

7212019 The Origin of High Efficiency in Low-temperature Solution

httpslidepdfcomreaderfullthe-origin-of-high-efficiency-in-low-temperature-solution 19

The origin of high effi

ciency in low-temperaturesolution-processable bilayer organometal halidehybrid solar cellsdagger

Shuangyong SunDaggera Teddy SalimDaggera Nripan Mathewsabc Martial Duchampd

Chris Boothroydd Guichuan Xinge Tze Chien Sumbce and Yeng Ming Lamabf

This work reports a study into the origin of the high efficiency in solution-processable bilayer solar cells

based on methylammonium lead iodide (CH3NH3PbI3) and [66]-phenyl-C61-butyric acid methyl ester

(PC61BM) Our cell has a power conversion efficiency (PCE) of 52 under simulated AM 15G irradiation

(100 mW cm2) and an internal quantum efficiency of close to 100 which means that nearly all the

absorbed photons are converted to electrons and are efficiently collected at the electrodes This implies

that the exciton diffusion charge transfer and charge collection are highly efficient The high excitondiffusion efficiency is enabled by the long diffusion length of CH3NH3PbI3 relative to its thickness

Furthermore the low exciton binding energy of CH3NH3PbI3 implies that exciton splitting at the

CH3NH3PbI3PC61BM interface is very efficient With further increase in CH3NH3PbI3 thickness a higher

PCE of 74 could be obtained This is the highest efficiency attained for low temperature solution-

processable bilayer solar cells to date

Broader context

Low-temperature solution-processable bilayer solar cells with their simple architecture provide an inexpensive and straightforward platform for device fabri-

cation without the necessity for extensive morphological optimization For efficient solar cells good light absorption accompanied by efficient conversion of

photons to electrons is critical This work shows that solar cells based on hybrid organicndashinorganic lead halide as the donor and [66]-phenyl-C61-butyric acid

methyl ester (PC61BM) as the acceptor are able to resolve the conicting lm thickness requirements of high absorption together with efficient exciton diff usion

As a result practically all the photons absorbed by the active layer can be conver ted to electrons

Introduction

Low-temperature solution-processable solar cells are becoming

more attractive as they off er a viable alternative to conventional

solar cells fabricated via vacuum-based or high-temperature

processes for large-scale high-throughput and cost-eff ective

manufacturing on exible plastic substrates Despite the

reported high performances solar cells based on ternary or

quaternary metal chalcogenides12 and dye-sensitized solar

cells3 generally require high-temperature processes ($400 C)

restricting their compatibility with low heat-resistant polymer

substrates By contrast organichybrid solar cells can be readily

fabricated at low temperatures45 Hybrid solar cells which

combine the advantages off ered by both organic and inorganic

materials are also highly versatile in terms of the device

structure they can adopt Depending on how the photoactive

layer is constructed two basic solar cell device architectures canbe obtained ie bulk heterojunction (BHJ) and bilayer hetero-

junction Bilayer solar cells have the merit of a more direct

charge transport pathway at the expense of poorer exciton

dissociation as compared to their BHJ counterpart Further-

more the bilayer solar cell with its simple architecture essen-

tially consisting of a stack of two photoactive layers provides an

inexpensive and straightforward platform for device fabrication

without the necessity for extensive morphological optimization

Very recently semiconductors with the perovskite structure

(ABX 3) have become a popular class of materials for solar cells

a Nanyang T echnological University School of Materials Science and Engineering 50

Nanyang Avenue 639798 Singapore E-mail ymlamntuedusg Fax +65 6790 9081b Energy Research Institute NTU (ERIN) 1 CleanTech Loop 06-04 CleanTech

One Singapore 637141 SingaporecSingapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) 1 Create

Way Singapore 138602 Singapored Ernst Ruska-Centrum und Peter Gr unberg Institut Forschungszentrum J ulich

D-52425 J ulich Germanye Division of Physics and Applied Physics School of Physical and Mathematical

Sciences Nanyang Technological University 21 Nanyang Link Singapore 637371

Singapore f Institute of Materials for Ele ctronic Engineering II RWTH-Aachen Sommerfeldstr 24

D-52074 Aachen Germany

dagger Electronic supplementary information (ESI) available See DOI

101039c3ee43161d

Dagger The authors contributed equally

Cite this Energy Environ Sci 2014 7399

Received 20th September 2013Accepted 23rd October 2013

DOI 101039c3ee43161d

wwwrscorgees

This journal is copy The Royal Society of Chemistry 2014 Energy Environ Sci 2014 7 399ndash407 | 399

Energy ampEnvironmentalScience

PAPER



Hybrid organicndashinorganic lead halide (eg CH3NH3PbX 3 X frac14

halogen) perovskites with good intrinsic optoelectronic prop-

erties have been synthesized and used for photovoltaic cell

applications6ndash11 These hybrid perovskites are also commonly

perhaps inappropriately referred to as organometal halide

trihalide The highest efficiency of perovskite solar cells to

date was 154 reported by Liu et al which has a planar het-

erojunction architecture with vapour-deposited perovskite

sandwiched between TiO2 and spiro-OMeTAD12 In anotherstudy Lee et al demonstrated that CH3NH3PbI2Cl in conjunc-

tion with spiro-OMeTAD could work as both a light absorber

and an electron conductor8 On the other hand Etgar et al

demonstrated that CH3NH3PbI3 can act simultaneously as an

efficient light harvester and transport holes in CH3NH3PbI3

TiO2 hybrid heterojunction solar cells9 It is also possible tone-

tune the bandgap of CH3NH3PbX 3 by varying the chemical

composition of the material resulting in a range of device

performance and stability10 Despite the great excitement in the

solar cell eld the application of CH3NH3PbX 3 for low-

temperature-processable solar cells is very limited at present

The previous reported works mostly employ mesoporous oxidelayers and hole blocking layers that require high-temperature

processing Hence an avenue towards the utilization of these

materials for low-temperature processable solar cells has to be

identied The pairing of CH3NH3PbX 3 with the other electron

acceptors other than inorganic metal oxides also has not been

much explored and hence investigation of alternative materials

may open up new possibilities in this area Bilayer devices

provide a straightforward platform to assess the compatibility

between CH3NH3PbX 3 and novel acceptor materials Further-

more due to the versatility of organometal halides in trans-

porting both holes and electrons it is possible that with the

right combination of materials bilayer devices based on orga-

nometal halides will perform efficiently Very recently Epersonet al demonstrated that a CH3NH3PbI3xClx based solar cell

containing no mesoporous layer with well controlled

morphology can achieve a PCE above 10 which shows planar

heterojunction perovskite solar cells have the advantage of

simple architecture hence ease of fabrication14

In this contribution we demonstrate the outstanding planar

device performance and investigate the origin of the high effi-

ciency in these solution-processable solar cells based on

methylammonium lead iodide (CH3NH3PbI3) and [66]-phenyl-

C61-butyric acid methyl ester (PC61BM) in a bilayer congura-

tion ITOPEDOTPSSCH3NH3PbI3PC61BMAl CH3NH3PbI3

and PC61BM behave as the electron donor (D) and acceptor (A)respectively This simple planar heterojunction solar cell

exhibits a remarkable performance giving a PCE of 523 with

a short-circuit current ( J SC) of 8201 mA cm2 an open-circuit

voltage (V OC) of 0824 V and a ll factor (FF) of 0774 under

simulated AM 15G irradiation (100 mW cm2) By employing a

thicker CH3NH3PbI3 layer a much higher PCE of 741 with J SCof 10829 mA cm2 V OC of 0905 V and FF of 0756 can be

obtained To date this is the most efficient low-temperature

solution-processable bilayer solar cell The high performance

of our device is found to be the result of the high internal

quantum efficiency (IQE) of close to 100 indicating that

almost every absorbed photon can be successfully converted to

free charge carriers that can be efficiently collected at the elec-

trodes Besides the appealing facile all-solution-processability

we note that our bilayer device is also ultrathin and can be

processed at low temperatures (lt150 C) rendering it compat-

ible with exible substrates and lightweight applications

Results and discussionMethylammonium lead iodide (CH3NH3PbI3) lm was formed via

spincoating of an equimolar mixture of CH3NH3I and PbI2precursor solutions The lm was heat-treated at 100 C and was

further characterized by X-ray diff raction (XRD) The appearance

of strong peaks at 2q frac14 1395 2835 and 3170 (ESI Fig S1dagger)

corresponding to the (110) (220) and (310) planes indicates the

formation of the tetragonal perovskite structure1516 A er verifying

that CH3NH3PbI3 was successfully synthesized we coupled it with

the n-type organic semiconductor PC61BM to make the bilayer

solar cells Fig 1 shows the device structure of the hybrid

CH3NH3PbI3PC61BM solar cell and the energy level diagram of

the respective device components As seen from the energy leveldiagram the combination of these two active materials yields

a type-II heterojunction The CH3NH3PbI3 lm was rst

deposited on poly(34-ethylenedioxythiophene)poly(styrenesulfonate)

(PEDOTPSS)-coated indium tin oxide (ITO) substrates and

was subsequently heated at 100 C for 30 s It is imperative to

note that heat treatment is essential for a complete conversion

from precursor to nal organolead halide It was found that

Fig 1 (a) Device structure of a CH3NH3PbI3PC61BM bilayer solar cell(b) Schematic energy level diagram of ITO PEDOTPSS CH3NH3PbI3PC61BM and Al The energy levels are taken from Kim et al6 andThompson amp Frechet13 The energy offset is noted at DE

400 | Energy Environ Sci 2014 7 399ndash407 This journal is copy The Royal Society of Chemistry 2014

Energy amp Environmental Science Paper



heating the lm at 100 C for as short as 30 s was sufficient to

achieve this purpose The heat-treated CH3NH3PbI3 lm

exhibits better optical absorption higher crystallinity and an

increase in interfacial area for exciton dissociation all of which

lead to an enhancement of device performance The eff ect of

heat treatment is discussed in detail in the ESIdagger Next the

PC61BM layer was spincoated onto the heat-treated CH3NH3PbI3layer The CH3NH3PbI3 and PC61BM are soluble in polar and

non-polar solvents respectively Solvent orthogonality ensuresthat the CH3NH3PbI3 lm remains intact a er PC61BM depo-

sition No heat treatment was applied a er coating with the

PC61BM lm to prevent interdiff usion between the two mate-

rials at the heterojunctions Eventually an aluminum cathode

was deposited on the active layer through a shadow mask Our

device fabrication methodology is highly compatible with low-

temperature processes emphasizing its potential for cost-

eff ective exible solar cells

Fig 2 shows a bright eld cross-sectional transmission

electron microscope (TEM) image of our device and a high

resolution TEM (HRTEM) image of the crystalline CH3NH3PbI3

layer The ultrathin cross-sectional sample was prepared using a

focused ion beam (FIB) In the TEM image (Fig 2a) distinct

layers with diff erent features and contrast can be observed

conrming that our fabricated devices indeed have a planar

junction architecture with a small donorndashacceptor interface

area While previous work on organolead halide only employs

this material deposited on inorganic meso-superstruc-

tures6ndash81011 we note that a standalone compact CH3NH3PbI3lm can also be implemented for solar cells Essentially our

CH3NH3PbI3 layer has great similarity to the solid organoleadhalide ldquocapping layerrdquo observed by Ball et al17 The thin layer of

CH3NH3PbI3 contains many dark spots corresponding to high

atomic number crystalline material The adjacent bright layers

below and above the CH3NH3PbI3 layer are PEDOTPSS and

PC61BM As observed PC61BM forms a continuous layer on

CH3NH3PbI3 and the interface between them is not diff use

Similar features with reversed contrast can be seen in the

annular dark-eld scanning transmission electron microscopy

(ADF-STEM) image (ESI Fig S2dagger) The HRTEM image of the

CH3NH3PbI3 layer (Fig 2b) reveals lattice fringes which corre-

spond to the planes from the intercalating PbI2 and CH3NH3I

surrounded by some amorphous regions The lattice planescorrespond to those observed by XRD (ESI Fig S1dagger) The darker

contrast observed could indicate either a few overlapping crys-

tallite planes or the existence of planes with diff erent orienta-

tions Unlike crystalline CH3NH3PbI3 quantum dots previously

reported to form on TiO2 surfaces1618 we note that our depo-

sition yields polycrystalline regions throughout the

CH3NH3PbI3 lm From Fig 2a it can be seen that both the

CH3NH3PbI3 and the PC61BM layers have a thickness of 50

5 nm which agrees well with the thickness measured using a

surface prolometer It is also noticeable that our CH3NH3PbI3

PC61BM bilayer is signicantly thinner than the previously

reported systems on organolead halide perovskite (gt300 nm) To

further conrm the material in each distinct layer and to verify that no interdiff usion occurs in the bilayer device EDS spectra

were collected from all the layers as shown in ESI Fig S3dagger

We note that the interface between PEDOTPSS and

CH3NH3PbI3 which is analogous to the other hybrid Schottky

diodes19ndash21 could potentially act as heterojunction for exciton

dissociation due to the existence of an internal electric eld at

the interface In that case the holes are transferred to the Fermi

level ( E F) of PEDOTPSS while the electrons are transferred to

the conduction-band minimum ( E C) of CH3NH3PbI3 In order to

investigate these possibilities we performed steady state pho-

toluminescence (PL) measurements on CH3NH3PbI3 only

CH3NH3PbI3PEDOTPSS and CH3NH3PbI3PC61BM bilayers Asindicated in ESI Fig S4dagger the PL intensity is quenched by a

factor of 4 when coupled with the PEDOTPSS layer and is

further quenched by an additional factor of 3 in the

CH3NH3PbI3PC61BM heterojunction This evidently indicates

the efficacy of both junctions in splitting the excitons generated

inCH3NH3PbI3 with PC61BM being a superior electron acceptor

which agrees with the recent nding by Abrusci et al22

To determine the feasibility of PC61BM as electron acceptor

in our planar heterojunction conguration we compared

CH3NH3PbI3 perovskite devices with and without a PC61BM

layer The devices were not based on our optimized fabrication

Fig 2 (a) Bright-1047297eld TEM cross-sectional image of the ITOPEDOTPSSCH3NH3PbI3PC61BMAl bilayer solar cell (b) HRTEMimage of the CH3NH3PbI3 layer The (310) and (224) lattice planes areidenti1047297ed in the inset The cross-section sample was prepared usingFIB


Paper Energy amp Environmental Science



conditions The current density ndash voltage ( J ndashV ) characteristics of

their respective devices are provided in ESI Fig S5dagger In the

absence of PC61BM the ITOPEDOTPSSCH3NH3PbI3Al device

does not exhibit any meaningful power that could be extracted

from the fourth quadrant of the light J ndashV characteristic graph

The fact that there is no signicant photovoltaic characteristic

observable in the ITOPEDOTPSSCH3NH3PbI3Al device

despite the reported ambipolar characteristics of organolead

halide and its use as an n-type transporter81723 suggests that either the PEDOTPSSCH3NH3PbI3 interface is less efficacious

in promoting exciton dissociation and charge transfer which

correlates with the PL data or the positioning of the electronic

levels of the device components is unfavorable for both charge

transport and collection On the other hand the device with the

PC61BM layer demonstrates typical photovoltaic behavior with a

short-circuit current ( J SC) of5961 mA cm2 open-circuit voltage

(V OC) of 0832 V ll factor (FF) of 0671 and power conversion

efficiency (PCE) of 333 The lack of efficient exciton dissoci-

ating interfaces in the absence of n-type PC61BM in one of the

devices compared to the other is responsible for the remarkable

diff

erence in device performance of the two types of devices

24

The thickness of CH3NH3PbI3 layers can be controlled by

varying the concentration of the organolead halide solution and

the deposition parameters The best performing device was

obtained using a 9 wt precursor solution and no interlayer

was applied between the PC61BM and the Al Fig 3a demon-

strates the J ndashV characteristics of our best CH3NH3PbI3PC61BM

bilayer solar cell device in dark and under AM 15G illumination

(100 mW cm2) The optimum device exhibits outstanding

performance with a J SC of 8201 mA cm2 V OC of 0824 V FF of

0774 and PCE of 523 To the best of our knowledge this is

one of the highest reported efficiencies for a low-temperature

solution-processable solar cell with a genuine bilayer architec-

ture During the course of our manuscript preparation Jeng et al reported a similar system with an efficiency of 39 and

with a lower FF and V OC25 attributable to the roughness of the

CH3NH3PbI3 lms The planarity of their interface was also not

veried

The high J SC measured in CH3NH3PbI3PC61BM which is

atypical of organichybrid bilayer devices could be an amalgam

of a few factors (1) a large optical absorption coefficient (2) a

wide absorption window (3) a moderate exciton diff usion

length and (4) excellent charge-carrier mobility We calculated

the absorption coefficient (a) of a CH3NH3PbI3 lm on a quartz

substrate based on the methodology developed by Cesaria

et al

26

which realistically accounts for the non-measurablecontribution of the lmndashsubstrate interface The calculated a

for the CH3NH3PbI3 perovskite lm is 43 105 cm1 at 360 nm

(ESI Fig S6dagger) which is approximately an order of magnitude

higher than the previously reported value by Im et al16 They

estimated the a of CH3NH3PbI3 to be 15 104 cm1 at 550 nm

while our calculation yields a to be 13 105 cm1 in the same

wavelength range The inconsistency in the a values could be

ascribed to the fact that Im et al performed their measurement

on CH3NH3PbI3-coated TiO2 samples and in that case the

measurement included the contribution of TiO2 The a of

CH3NH3PbI3 is comparable to or even higher than many

conjugated molecules commonly used in highly efficient

organic solar cell devices27ndash29 As compared to inorganic semi-conductors eg Cu2ZnSnS4 (az 61 104 cm1 at 650 nm)30 or

CuInSe2 (afrac14 6 105 cm1 at 690 nm)31 CH3NH3PbI3 has a very

similar range of absorption coefficient Due to the reasonably

high a ofCH3NH3PbI3 in the visible region it is possible to form

a sufficiently thin yet strongly absorbing lm which is crucial to

a bilayer device

Furthermore the optical bandgap of CH3NH3PbI3( E g z 15 eV) corresponds well to the bandgap requirement of

an ideal single pndashn junction solar cell32 The absorption of the

organolead halide perovskite extends throughout the UV-visible

region with an absorption onset at ca 790 nm (Fig 4a) The

CH3NH3PbI3

lm exhibits an absorption peak at ca 360 nm anda ldquoshoulderrdquo band at ca 480 nm On the other hand the

CH3NH3PbI3PC61BM bilayer in which both layers have a

similar range of thickness exhibits superposed absorption

characteristics of its constituents The broad absorption

window overlapping with the maximum irradiance of the solar

spectrum ensures efficient photon harvesting which may

eventually lead to a high photocurrent

Fig 3b shows the active absorption external quantum effi-

ciency (EQE) and internal quantum efficiency (IQE) curves from

our best CH3NH3PbI3PC61BM device The absorption was

measured directly on an ITOPEDOTPSSCH3NH3PbI3PC61BM

Fig 3 (a) Current densityndashvoltage (JndashV ) characteristics of ITOPEDOTPSSCH3NH3PbI3PC61BMAl bilayer solar cell in dark andunderAM 15G illumination (b) EQE IQE and active absorption spectraof the ITOPEDOTPSSCH3NH3PbI3PC61BMAl bilayer solar cell





Al bilayer device based on the technique proposed by Burkhard

et al33 as detailed in the ESIdagger The parasitic absorption spectra

and the derivation of the active layer absorption are shown in ESI

Fig S7dagger The CH3NH3PbI3PC61BM absorption characteristics

obtained from the device via the semi-empirical approach

slightly diff er from those acquired from the bilayer lm alone

(Fig 4a) It is plausible that optical interference eff ects induce

the wavelength-dependent exciton generation proles to have

maxima at diff erent spots in the device (ESI Fig S8dagger) Conse-

quently the fraction of photoabsorption in each layer is

diff erent attributed to the alteration of the ldquoeff ectiverdquo absorp-

tion of the bilayer As shown in Fig 3b the CH3NH3PbI3PC61BM

device exhibits an EQE (charge extracted per incident photon) as

high as 41 at ca 570 nm The integration of EQE spectra yields

J SC of 804 mA cm2 approximately lt2 diff erent from the value

measured under AM 15G illumination The measurement of IQE ie charge extracted per photon absorbed provides some

insight into the possible operating mechanism of our

CH3NH3PbI3PC61BM bilayer device and the origin of the high

PCE thereof It decouples the contribution of photon absorption

and is a practical gauge in assessing the eff ectiveness of bilayer

architecture to generate photocurrent The IQE can be written as

hIQE frac14 hED hCT hCC (1)

where hED is exciton diff usion efficiency hCT is charge transfer

efficiency and hCC is charge collection efficiency The IQE is

conventionally larger than the EQE which is also observed in our

bilayer device (Fig 3b) and was calculated by considering both the

EQE and the active absorption The IQE of the CH3NH3PbI3

PC61BM bilayer device is above 90 throughout the wavelength

range of 550ndash650 nm with an IQEmax of 95 at ca 580 nm

Interestingly this wavelength range corresponds to the local

maxima of the electriceldsin the CH3NH3PbI3 layer as shown in

our simulation results (ESI Fig S9dagger) In general a highIQE gt 80

is observable in nearly the entire visible spectrum (490ndash750 nm)Such a remarkable IQE (approaching unity) indicates that almost

every absorbed photon is converted into a pair of charge-carriers

and that almost all charge-carriers generated are efficiently

collected at both electrodes Ball et al have also demonstrated an

IQE close to 100 for their Cl-doped CH3NH3PbI3 solar cells

although their IQE was not derived directly from the EQE and was

instead estimated from the device photocurrent ( J SC)17

The surprisingly high IQE in our bilayer device can be

explained by evaluating the three factors that IQE is dependent

on Firstly the exciton diff usion length ( LD) is comparable to the

thickness of the perovskite layer ($50 nm) In comparison the

LD in conjugated polymers is reported to be in the range of 2ndash

10 nm34ndash36 Besides the exciton lifetime (s) in pure CH3NH3PbI3powder is exceptionally high up to 78 ns6 while s for singlet

excitons in organic semiconductors is much shorter (lt5 ns)37

The combination of these two eff ects means that the excitons in

the CH3NH3PbI3 lm can travel a longer distance before decay

increasing their likelihood of reaching the heterojunction to

dissociate into free electronndashhole pairs

Secondly there is an efficient charge transfer from the

CH3NH3PbI3 to the PC61BM at the interface This is evident from

the efficient photoluminescence (PL) quenching of CH3NH3PbI3(gt90) in the presence of PC61BM (Fig 4b) The efficient PL

quenching at the CH3NH3PbI3PC61BM interface which is

comparable to that observed in nanoscale phase-separatedpolymerndashfullerene BHJs3839 is indicative of an efficient exciton

diff usion in CH3NH3PbI3 to the dissociating interface followed

by an efficient electron transfer to the fullerene phase The

exciton binding energy ( E b) which greatly impacts the efficiency

of exciton dissociation and charge transfer was elucidated

through temperature-dependent photoluminescence (PL)

Fig 4 (a) UV-vis absorption spectra from the CH3NH3PbI3PC61BMbilayer and its constituents The AM 15G spectral irradiance is alsoincluded for comparison (b) Steady-state PL spectra for CH3NH3PbI3and CH3NH3PbI3PC61BM (lex frac14 600 nm)

Fig 5 Temperature-dependent integrated PL intensity of theCH3NH3PbI3 1047297lm under excitation of a 532 nm continuous-wave laserbeam The solid line is the best 1047297t based on the Arrhenius equation





measurements From the plot of the integrated PL intensity as a

function of temperature (Fig 5) a least square t of the data with

the Arrhenius equation ( I (T )frac14 I 0(1 + Ae( E bk BT ))) yields an exciton

binding energyof 19 3 meV This indicates that an electric eld

is still required to dissociate all the excitonsgenerated but as this

value is much lower than that of typical organic semiconductors

(gt100 meV)40 the electric eld required to split the excitons is

much lower The overall driving force to promote exciton disso-

ciation and electron transfer from the CH3NH3PbI3 donor to thePC61BM acceptor is provided by the energy off set between the

conduction-band minimum ( E C) of CH3NH3PbI3 and the lowest-

unoccupied molecular orbital (LUMO) of PC61BM From the

energy level diagram (Fig 1b) it can be seen that the energy

off set (D E frac14 027 eV) is more than 10-fold higher than the E b of

CH3NH3PbI3 suggesting that the internal eld at the hetero-

interface is indisputably sufficient for exciton dissociation This

results in a nearly instantaneous exciton splitting at the interface

preceding the electron transfer to the fullerene phase Further-

more the deep LUMO level of C60 derivatives tends to induce

high electron affinity towards electron donating materials

promoting a favorable charge transfer

41

The holes inCH3NH3PbI3 and the electrons in PC61BM eventually dri

towards the anode (ITO) and cathode (Al) respectively

Thirdly it is important to consider the long-range charge

transport and efficient charge collection The subsequent

charge transport is also not an issue as the pathways of holes to

the anode and electrons to the cathode are well dened and

continuous On top of that the high hole mobility (mhz 06 cm2

V 1 s1)42 of metal halide perovskite and the high electron

mobility (mez 1 cm2 V 1 s1)43 of PC61BM have been reported

based on eld-eff ect transistor measurements Ball et al have

also observed eff ective charge transport and collection in their

CH3NH3PbX 3 devices in which the thick organolead halide

layer did not adversely aff ect the device performance17 There-fore the charge recombination in our CH3NH3PbI3PC61BM

bilayer device is expected to be low

Besides a high J SC the other two factors responsible for the

highly efficient CH3NH3PbI3PC61BM bilayer device are its

impressive V OC and FF The high V OC of 0824 V is comparable to

those reported for ldquochampionrdquo organic solar cells based on low

bandgap materials eg PTB7PCBM (V OC frac14 074 V)44 PBDTTT-

CFPCBM (V OC frac14 076 V)45 and PCDTBTPCBM (V OC frac14 088

V)28 In comparison wide bandgap P3HT only gives a V OC of

06 V when coupled with PCBM46 Coincidentally the V OC of

CH3NH3PbI3PC61BM lines up agreeably in comparison with the

abovementioned devices following the monotonic trend of highest-occupied molecular orbital (HOMO) or valence-band

maximum ( E V ) levels of the donor materials (ie 5 eV

515 eV 522 eV 543 eV and 55 eV for P3HT PTB7

PBDTTT-CF CH3NH3PbI3 and PCDTBT respectively) It has

been reported that the V OC of blend organic solar cells is

empirically governed by the following relationship

V OCz (1e)(|E donorHOMO| |E acceptorLUMO |) (2)

where E donorHOMO is the HOMO level of the donor and E acceptorLUMO is the

LUMO level of the acceptor47 Since CH3NH3PbI3 has a

deep-lying E V (or HOMO-equivalent) level the device is expected

to yield a high photovoltage Our result also emphasizes the

feasibility of estimating the V OC of both metal halide perovskite

fullerene bilayers and blend devices

Our CH3NH3PbI3PC61BM bilayer solar cell has a magni-

cent FF of up to 0774 and is among the highest reported for

solution-deposited solar cells This value is remarkable as

highly efficient solution-processable solar cells with well-

engineered electrode interfaces so far have only shown FF values of 060ndash07244548 By contrast we did not apply any

electron-transport layer (ETL) to assist charge collection The

high FF indicates that the charge transport and collection of the

bilayer solar cells are very efficient corresponding well with the

high IQE It also implies that the charge recombination in this

bilayer system is not an efficiency-limiting issue It is well

known that the FF parameter is closely related to both the series

resistance ( Rs) and the shunt resistance ( Rsh) of solar cells From

the slope of the J ndashV curve around the open-circuit and short-

circuit regions Rs and Rsh are calculated to be 64 U cm2 and

16 k U cm2 respectively The low Rs is usually associated with

effi

cient charge transport with negligible charge accumulationand recombination

Throughout this contribution we have shown that in our

bilayer solar cell device based on an ITOPEDOTPSS

CH3NH3PbI3PC61BMAl conguration CH3NH3PbI3 works as

the main light absorber electron donor (D) and p-type (hole)

conductor while PC61BM acts as the electron acceptor (A) and

n-type (electron) conductor The photogenerated excitons in the

CH3NH3PbI3 layer get dissociated at the DA interfaces The

holes in the CH3NH3PbI3 and electrons in the PC61BM eventu-

ally dri towards and are collected at the anode (ITO) and

cathode (Al) respectively The PC61BM also helps in preventing

a direct contact between the highly conductive CH3NH3PbI3

(103 S cm3)9 and the top electrode The PEDOTPSS layer

essentially smoothens the ITO surface and assists the hole

transport from CH3NH3PbI3 to ITO It is also demonstrated that

a PCE in excess of 5 could be obtained from the bilayer

CH3NH3PbI3PC61BM device In comparison solution-

deposited organic solar cells with discernible planar inter-

faces usually suff er from poor PCEs (lt2) attributed to the

short exciton diff usion length4950 It is necessary to diff erentiate

between pure bilayer and ldquopseudordquo bilayer the latter manifests

from either a thermally driven interdiff usion between both

layers51ndash53 or a simple sandwiching of a heterogeneous layer

between two homogeneous layers5455 Due to the diff erence in

device geometry ie a lack of well-de

ned planar hetero-interfaces ldquopseudordquo bilayer devices are not comparable with

these CH3NH3PbI3PC61BM bilayer devices

Despite the remarkable performance it is imperative to

point out that the CH3NH3PbI3 absorption in our bilayer system

is yet to be optimized As seen earlier the EQE obtained from

our CH3NH3PbI3PC61BM bilayer device is noticeably lower than

the reported values for the other organolead halide perovskite

devices (EQE gt 60)7810 and this likely due to our poorer

perovskite absorption We surmise that the enhancement in

optical absorption should lead to higher EQE and hence J SC To

attest this hypothesis similar devices were prepared with





thicker CH3NH3PbI3 lms (110 5 nm) on PEDOTPSS

following the procedures reported by Burschka et al11 This

approach involves two-step deposition of each organolead

halide precursor as opposed to the one-step deposition of the

mixture of precursors previously employed A signicant

improvement in photocurrent was observed as shown in Fig 6a

The device showed a J SC of 10829 mA cm2 V OC of 0905 VFF of

0756 and PCE of 741 This is nearly twice as high as the

results obtained by Jeng et al with a comparable device struc-ture25 The higher J SC is also conrmed by a much higher EQE of

65 at ca 550 nm (Fig 6b) Liu et al reported J SC in excess of

20 mA cm2 with an organolead halide layer of more than

300 nm12 Therefore further enhancement in J SC which is the

pivotal parameter in our PCE improvement can be expected for

our bilayer system if the main absorber layer ie CH3NH3PbI3

is made even thicker Besides absorption optimization it will

also be interesting to couple organolead halide perovskite with

the other types of fullerene derivatives with either better

absorption characteristics to induce higher photocurrent or

better energetics to improve photovoltage Lastly for long-term

practical use of bilayer devices they should be subjected to

stability tests to validate their reliability

Conclusions

In summary we have demonstrated a pure bilayer CH3NH3PbI3

PC61BM solar cell which has excellent performance with a PCE

of 74 the best reported to date for an all-solution-processable

planar heterojunction device The active layer is sufficiently thin(lt100 nm) and the device can be fabricated with low-

temperature processes (lt150 C) Furthermore a bilayer

system is also desirable due to its simplicity in processing The

high performance of the bilayer device can be attributed to the

high IQE of close to unity (100) suggesting that the exciton

diff usion charge transfer and charge collection are highly effi-

cient The high ll factor of 77 is also one of the best reported

for organic and hybrid solar cells Through further optimization

of photon harvesting an improvement in PCE to beyond 10 is

within reach

Experimental sectionMaterials

Methylammonium iodide (CH3NH3I) was synthesized accord-

ing to reported procedures16 To prepare the organolead halide

perovskite precursor solution as-synthesized CH3NH3I powder

and lead(II) iodide powder (PbI2 Aldrich) were mixed in anhy-

drous dimethylformamide (DMF Aldrich) with a weight ratio of

1 3 The suspension (9 wt) was stirred overnight at 60 C

Prior to device fabrication the precursor solution was ltered

with a 045 mm PVDF lter [66]-Phenyl-C61-butyric acid methyl

ester (PC61BM) was purchased from Nano-Creg PC61BM

(10 mg mL1) was dissolved in a solvent mixture of anhydrous

chlorobenzene (CB Aldrich) and anhydrous chloroform (CF

Aldrich) (CB CF frac14 1 1 vv) All materials were used directly

without purication

Device fabrication and characterization

Solar cells were fabricated on indium tin oxide (ITO)-coated

glass substrates (Xinyan Technology Ltd 7 U sq1) with the

following device conguration ITOPEDOTPSSCH3NH3PbI3

PC61BMAl The ITO-coated glass substrates were successively

cleaned in detergent deionized water acetone and isopropanol

in an ultrasonic bath for 15 min each Subsequently the

substrates were plasma-cleaned for 2 min before being coated with a 30 nm-thick PEDOTPSS (Cleviostrade Al 4083) layer

A erwards the substrates were baked at 140 C for 10 min in a

N2 lled glove box Perovskite precursor solution was spin-

coated on the PEDOTPSS layer at 3000 rpm for 40 s The

lms were subsequently heated at 100 C for 30 s The

CH3NH3PbI3 samples were always subjected to thermal treat-

ment by default unless other processing conditions are speci-

ed The PC61BM layer was then spin-coated onto the

CH3NH3PbI3 layer at 1200 rpm for 60 s to generate the bilayer

No heat treatment was done on the PC61BM layer Finally an

aluminum cathode (100 nm) was deposited on the active layer

Fig 6 (a) Current densityndashvoltage (JndashV ) characteristics of ITOPEDOTPSSCH3NH3PbI3PC61BMAl bilayer solar cell with thickerCH3NH3PbI3 in dark andunder AM 15G illumination (b) EQEspectra of

the corresponding ITOPEDOTPSSCH3NH3PbI3PC61BMAl bilayersolar cell





through a shadow mask to give a device area of 007 cm2 under a

vacuum level of 106 torr Approximately 20 devices were

fabricated for each experimental variable

The current density ndash voltage ( J ndashV ) characteristics of the devices

were measured in darkness and under Air Mass 15 Global (AM

15G) illumination (SAN-EI Electric) using a Keithley SMU 2400

source meter The light intensity was rst calibrated to 100 mW

cm2 with a digital Solar Meter (Daystar DS-05A) External

quantum efficiency (EQE) measurement was done with a Merlinradiometer (Newport) with a monochromator-calibrated wave-

length control Thelight was coupled into an optical cable (Ocean

Optics) A calibrated silicon photodiode (Hamamatsu) was used

as a reference device in counting incident photons All device

measurements were performed in an inert N2 environment

Materials characterization

Optical absorbance spectra of perovskite lms on quartz and

total reectance of the device were measured using a UV-vis-NIR

spectrophotometer (Shimadzu UV-3600) with an integrating

sphere (ISR-3100) The internal quantum effi

ciency (IQE) of thedevice was calculated according to the reported literature33 The

total absorption of the device was calculated from the measured

reectance spectrum of the device Parasitic absorption was

calculated using a transfer matrix formalism to evaluate the

absorptions not in the active layer For the modeling the

refractive indices (n k ) of the materials were either obtained

from the literature or measured with an ellipsometer The active

absorption from the active layer was obtained by subtracting the

modeled parasitic absorption from the total absorption A er-

wards the IQE was calculated based on the measured EQE and

the active absorption

The crystal structure of the CH3NH3PbI3 perovskite lm was

investigated with an X-ray diff ractometer (XRD Bruker D8 Advance) equipped with a Cu-K

a X-ray tube The acquisition was

carried out in the range 10ndash60 in qndash2q mode with a scan angle

of 1 using a step size of 004 and a time step of 3 s The surface

morphology of the perovskite was imaged with tapping-mode

atomic force microscopy (AFM Asylum MFP-3D-BIO) An Al

reex coated AFM probe (Olympus AC240TS) with a spring

constant of 2 N m1 and tip radius of 9 nm was used TEM

cross-section samples were prepared using a standard li -out

procedure in a dual-beam FEI Helios focused ion beam (FIB)

workstation Two consecutive protective layers of 100 nm of

electron-beam-deposited Pt and 1 mm of ion-beam-deposited

Pt were used to avoid degradation of the samples due to Ga2+

implantation during milling Coarse FIB milling was carried out

using a 30 kV ion beam while nal milling was performed at

5 kV ion energy The angular dark-eld scanning transmission

electron microscope (ADF-STEM) imaging was performed in a

FEI Helios electron microscope (15 kV FEG) and high-resolution

transmission electron microscope (HR-TEM) images were

obtained using a Philips CM20 TEM (200 kV FEG) and a FEI

Titan (300 kV FEG) The EDX characterization was performed

using a Philips CM20 TEM and an EDAX detector The thickness

of the CH3NH3PbI3 lm was measured with both an Alpha-Step

proler (KLA-Tencor) and an AFM

For time-integrated photoluminescence (PL) measurements

the light source was a Coherent Legendtrade regenerative ampli-

er (150 fs 1 kHz 800 nm) that was seeded by a Coherent

Vitessetrade oscillator (100 fs 80 MHz) 600 nm 150 fs laser pulses

were generated from a Coherent TOPAS-C optical parametric

amplier The laser pulses were directed to the lms which were

put in vacuum The uence was kept to a minimum of 13 m J

cm2 per pulse to avoid any second order eff ects in the

dynamics The emission from the samples was collected at abackscattering angle of 150 by a pair of lenses and into an

optical ber that was coupled to a spectrometer (Acton Spectra

Pro 2500i) to be detected by a charge-coupled device (Princeton

Instruments Pixis 400B) To determine the exciton binding

energy the temperature dependent PL was conducted with the

same experimental geometry but with a continuous excitation

light source (532 nm 1 mJ cm2)

Acknowledgements

The authors also would like to especially thank Dr Eric T Hoke

for discussion on the transfer matrix calculation Drs ChuanSeng Tan and Kwang Hong Lee for the ellipsometry measure-

ment and Ms Jeannette M Kadro for her help in the synthesis of

CH3NH3I The authors acknowledge the Energy Research

Institute NTU (ERIN) for the use of the facilities MD

acknowledges nancial support from the European Union

under the Seventh Framework Programme under a contract for

an Integrated Infrastructure Initiative (reference 312483 ndash

ESTEEM2) TCS acknowledges the support from the following

research grants NTU start-up grant (M58110068) a Competitive

Research Programme grant (NRF-CRP5-2009-04) and the

Singapore-Berkeley Research Initiative for Sustainable Energy

(SinBeRISE) CREATE Programme YML acknowledges the

support from The Danish Council for Strategic Research

References

1 Q Guo G M Ford R Agrawal and H W Hillhouse Progress

in Photovoltaics Research and Applications 2013 21 64ndash71

2 T K Todorov J Tang S Bag O Gunawan T Gokmen

Y Zhu and D B Mitzi Adv Energy Mater 2013 3 34ndash38

3 A Yella H-W Lee H N Tsao C Yi A K Chandiran

M K Nazeeruddin E W-G Diau C-Y Yeh

S M Zakeeruddin and M Gratzel Science 2011 334 629ndash

634

4 Z He C Zhong S Su M Xu H Wu and Y Cao Nat Photonics 2012 6 591ndash595

5 J You L Dou K Yoshimura T Kato K Ohya T Moriarty

K Emery C-C Chen J Gao G Li and Y Yang Nat

Commun 2013 4 1446

6 H-S Kim C-R Lee J-H Im K-B Lee T Moehl

A Marchioro S-J Moon R Humphry-Baker J-H Yum

J E Moser M Gratzel and N-G Park Sci Rep 2012 2 1ndash7

7 J H Heo S H Im J H Noh T N Mandal C-S Lim

J A Chang Y H Lee H-j Kim A Sarkar

K NazeeruddinMd M Gratzel and S I Seok Nat

Photonics 2013 7 486ndash491





8 M M Lee J Teuscher T Miyasaka T N Murakami and

H J Snaith Science 2012 338 643ndash647

9 L Etgar P Gao Z Xue Q Peng A K Chandiran B Liu

M K Nazeeruddin and M Gratzel J Am Chem Soc 2012

134 17396ndash17399

10 J H Noh S H Im J H Heo T N Mandal and S I Seok

Nano Lett 2013 13 1764ndash1769

11 J Burschka N Pellet S-J Moon R Humphry-Baker P Gao

M K Nazeeruddin and M Gratzel Nature 2013 499 316ndash

319

12 M Liu M B Johnston and H J Snaith Nature 2013 501

395ndash398

13 B C Thompson and J M J Frechet Angew Chem Int Ed

2008 47 58ndash77

14 G E Eperon V M Burlakov P Docampo A Goriely and

H J Snaith Adv Funct Mater 2013 DOI 101002

adfm201302090

15 A Kojima K Teshima Y Shirai and T Miyasaka J Am

Chem Soc 2009 131 6050ndash6051

16 J-H Im C-R Lee J-W Lee S-W Park and N-G Park

Nanoscale 2011 3

4088ndash

409317 J M Ball M M Lee A Hey and H J Snaith Energy Environ

Sci 2013 6 1739ndash1743

18 J-H Im J Chung S-J Kim and N-G Park Nanoscale Res

Lett 2012 7 353

19 S-C Shiu J-J Chao S-C Hung C-L Yeh and C-F Lin

Chem Mater 2010 22 3108ndash3113

20 F Zhang T Song and B Sun Nanotechnology 2012 23

194006

21 J-J Chao S-C Shiu S-C Hung and C-F Lin

Nanotechnology 2010 21 285203

22 A Abrusci S D Stranks P Docampo H-L Yip A K Y Jen

and H J Snaith Nano Lett 2013 13 3124ndash3128

23 E Edri S Kirmayer D Cahen and G Hodes J Phys Chem Lett 2013 4 897ndash902

24 C W Tang Appl Phys Lett 1986 48 183ndash185

25 J-Y Jeng Y-F Chiang M-H Lee S-R Peng T-F Guo

P Chen and T-C Wen Adv Mater 2013 25 3727ndash

3732

26 M Cesaria A P Caricato and M Martino J Opt 2012 14

105701

27 B Walker C Kim and T-Q Nguyen Chem Mater 2010 23

470ndash482

28 S H Park A Roy S Beaupre S Cho N Coates J S Moon

D Moses M Leclerc K Lee and A J Heeger Nat Photonics

2009 3

297ndash

30229 Y Kim S Cook S M Tuladhar S A Choulis J Nelson

J R Durrant D D C Bradley M Giles I McCulloch

C-S Ha and M Ree Nat Mater 2006 5 197ndash203

30 K Ito and T Nakazawa Jpn J Appl Phys 1988 27 2094ndash

2097

31 L L Kazmerski M Hallerdt P J Ireland R A Mickelsen

and W S Chen J Vac Sci Technol A 1983 1 395ndash398

32 W Shockley and H J Queisser J Appl Phys 1961 32 510ndash

519

33 G F Burkhard E T Hoke and M D McGehee Adv Mater

2010 22 3293ndash3297

34 D E Markov C Tanase P W M Blom and J Wildeman

Phys Rev B Condens Matter Mater Phys 2005 72 045217

35 J E Kroeze T J Savenije M J W Vermeulen and

J M Warman J Phys Chem B 2003 107 7696ndash7705

36 P E Shaw A Ruseckas and I D W Samuel Adv Mater

2008 20 3516ndash3520

37 M R Narayan and J Singh Phys Status Solidi C 2012 92386ndash2389

38 M Hallermann I Kriegel E Da Como J M Berger E von

Hauff and J Feldmann Adv Funct Mater 2009 19 3662ndash

3668

39 H Wang H-Y Wang B-R Gao L Wang Z-Y Yang

X-B Du Q-D Chen J-F Song and H-B Sun Nanoscale

2011 3 2280ndash2285

40 V I Arkhipov and H Bassler Phys Status Solidi A 2004 201

1152ndash1187

41 P M Allemand A Koch F Wudl Y Rubin F Diederich

M M Alvarez S J Anz and R L Whetten J Am Chem

Soc 1991 113

1050ndash

105142 C R Kagan D B Mitzi and C D Dimitrakopoulos Science

1999 286 945ndash947

43 T B Singh N Marjanovic G J Matt S Gunes

N S Sarici ci A Montaigne Ramil A Andreev H Sitter

R Schw odiauer and S Bauer Org Electron 2005 6 105ndash

110

44 Y Liang Z Xu J Xia S-T Tsai Y Wu G Li C Ray and

L Yu Adv Mater 2010 22 E135ndashE138

45 H-Y Chen J Hou S Zhang Y Liang G Yang Y Yang

L Yu Y Wu and G Li Nat Photonics 2009 3 649ndash653

46 G Li V Shrotriya J Huang Y Yao T Moriarty K Emery

and Y Yang Nat Mater 2005 4 864ndash868

47 M C Scharber D Muhlbacher M Koppe P DenkC Waldauf A J Heeger and C J Brabec Adv Mater

2006 18 789ndash794

48 C E Small S Chen J Subbiah C M Amb S-W Tsang

T-H Lai J R Reynolds and F So Nat Photonics 2012 6

115ndash120

49 A L Ayzner C J Tassone S H Tolbert and B J Schwartz

J Phys Chem C 2009 113 20050ndash20060

50 K H Lee P E Schwenn A R G Smith H Cavaye

P E Shaw M James K B Krueger I R Gentle

P Meredith and P L Burn Adv Mater 2011 23 766ndash770

51 V S Gevaerts L J A Koster M M Wienk and

R A J Janssen ACS Appl Mater Interfaces 2011 3

3252ndash

3255

52 A Loiudice A Rizzo M Biasiucci and G Gigli J Phys Chem

Lett 2012 3 1908ndash1915

53 D Chen F Liu C Wang A Nakahara and T P Russell


54 J Xue B P Rand S Uchida and S R Forrest Adv Mater

2005 17 66ndash71

55 X Xiao J D Zimmerman B E Lassiter K J Bergemann

and S R Forrest Appl Phys Lett 2013 102 073302





Hybrid organicndashinorganic lead halide (eg CH3NH3PbX 3 X frac14

halogen) perovskites with good intrinsic optoelectronic prop-

erties have been synthesized and used for photovoltaic cell

applications6ndash11 These hybrid perovskites are also commonly

perhaps inappropriately referred to as organometal halide

trihalide The highest efficiency of perovskite solar cells to

date was 154 reported by Liu et al which has a planar het-

erojunction architecture with vapour-deposited perovskite

sandwiched between TiO2 and spiro-OMeTAD12 In anotherstudy Lee et al demonstrated that CH3NH3PbI2Cl in conjunc-

tion with spiro-OMeTAD could work as both a light absorber

and an electron conductor8 On the other hand Etgar et al

demonstrated that CH3NH3PbI3 can act simultaneously as an

efficient light harvester and transport holes in CH3NH3PbI3

TiO2 hybrid heterojunction solar cells9 It is also possible tone-

tune the bandgap of CH3NH3PbX 3 by varying the chemical

composition of the material resulting in a range of device

performance and stability10 Despite the great excitement in the

solar cell eld the application of CH3NH3PbX 3 for low-

temperature-processable solar cells is very limited at present

The previous reported works mostly employ mesoporous oxidelayers and hole blocking layers that require high-temperature

processing Hence an avenue towards the utilization of these

materials for low-temperature processable solar cells has to be

identied The pairing of CH3NH3PbX 3 with the other electron

acceptors other than inorganic metal oxides also has not been

much explored and hence investigation of alternative materials

may open up new possibilities in this area Bilayer devices

provide a straightforward platform to assess the compatibility

between CH3NH3PbX 3 and novel acceptor materials Further-

more due to the versatility of organometal halides in trans-

porting both holes and electrons it is possible that with the

right combination of materials bilayer devices based on orga-

nometal halides will perform efficiently Very recently Epersonet al demonstrated that a CH3NH3PbI3xClx based solar cell

containing no mesoporous layer with well controlled

morphology can achieve a PCE above 10 which shows planar

heterojunction perovskite solar cells have the advantage of

simple architecture hence ease of fabrication14

In this contribution we demonstrate the outstanding planar

device performance and investigate the origin of the high effi-

ciency in these solution-processable solar cells based on

methylammonium lead iodide (CH3NH3PbI3) and [66]-phenyl-

C61-butyric acid methyl ester (PC61BM) in a bilayer congura-

tion ITOPEDOTPSSCH3NH3PbI3PC61BMAl CH3NH3PbI3

and PC61BM behave as the electron donor (D) and acceptor (A)respectively This simple planar heterojunction solar cell

exhibits a remarkable performance giving a PCE of 523 with

a short-circuit current ( J SC) of 8201 mA cm2 an open-circuit

voltage (V OC) of 0824 V and a ll factor (FF) of 0774 under

simulated AM 15G irradiation (100 mW cm2) By employing a

thicker CH3NH3PbI3 layer a much higher PCE of 741 with J SCof 10829 mA cm2 V OC of 0905 V and FF of 0756 can be

obtained To date this is the most efficient low-temperature

solution-processable bilayer solar cell The high performance

of our device is found to be the result of the high internal

quantum efficiency (IQE) of close to 100 indicating that

almost every absorbed photon can be successfully converted to

free charge carriers that can be efficiently collected at the elec-

trodes Besides the appealing facile all-solution-processability

we note that our bilayer device is also ultrathin and can be

processed at low temperatures (lt150 C) rendering it compat-

ible with exible substrates and lightweight applications

Results and discussionMethylammonium lead iodide (CH3NH3PbI3) lm was formed via

spincoating of an equimolar mixture of CH3NH3I and PbI2precursor solutions The lm was heat-treated at 100 C and was

further characterized by X-ray diff raction (XRD) The appearance

of strong peaks at 2q frac14 1395 2835 and 3170 (ESI Fig S1dagger)

corresponding to the (110) (220) and (310) planes indicates the

formation of the tetragonal perovskite structure1516 A er verifying

that CH3NH3PbI3 was successfully synthesized we coupled it with

the n-type organic semiconductor PC61BM to make the bilayer

solar cells Fig 1 shows the device structure of the hybrid

CH3NH3PbI3PC61BM solar cell and the energy level diagram of

the respective device components As seen from the energy leveldiagram the combination of these two active materials yields

a type-II heterojunction The CH3NH3PbI3 lm was rst

deposited on poly(34-ethylenedioxythiophene)poly(styrenesulfonate)

(PEDOTPSS)-coated indium tin oxide (ITO) substrates and

was subsequently heated at 100 C for 30 s It is imperative to

note that heat treatment is essential for a complete conversion

from precursor to nal organolead halide It was found that

Fig 1 (a) Device structure of a CH3NH3PbI3PC61BM bilayer solar cell(b) Schematic energy level diagram of ITO PEDOTPSS CH3NH3PbI3PC61BM and Al The energy levels are taken from Kim et al6 andThompson amp Frechet13 The energy offset is noted at DE




































































































diff


24
















veried








et al

26


















a bilayer device





CH3NH3PbI3







































































































41




















































effi






























































Conclusions













within reach














without purication






























































































Acknowledgements















References








634




Commun 2013 4 1446
















134 17396ndash17399





319


395ndash398


2008 47 58ndash77



adfm201302090


Chem Soc 2009 131 6050ndash6051


Nanoscale 2011 3

4088ndash


Sci 2013 6 1739ndash1743


Lett 2012 7 353




194006









3732


105701


470ndash482



2009 3

297ndash





2097




519


2010 22 3293ndash3297






2008 20 3516ndash3520




3668



2011 3 2280ndash2285


1152ndash1187



Soc 1991 113

1050ndash


1999 286 945ndash947




110








2006 18 789ndash794



115ndash120








3252ndash

3255


Lett 2012 3 1908ndash1915




2005 17 66ndash71






































































































diff


24
















veried








et al

26


















a bilayer device





CH3NH3PbI3







































































































41




















































effi






























































Conclusions













within reach














without purication






























































































Acknowledgements















References








634




Commun 2013 4 1446
















134 17396ndash17399





319


395ndash398


2008 47 58ndash77



adfm201302090


Chem Soc 2009 131 6050ndash6051


Nanoscale 2011 3

4088ndash


Sci 2013 6 1739ndash1743


Lett 2012 7 353




194006









3732


105701


470ndash482



2009 3

297ndash





2097




519


2010 22 3293ndash3297






2008 20 3516ndash3520




3668



2011 3 2280ndash2285


1152ndash1187



Soc 1991 113

1050ndash


1999 286 945ndash947




110








2006 18 789ndash794



115ndash120








3252ndash

3255


Lett 2012 3 1908ndash1915




2005 17 66ndash71





























































































41




















































effi






























































Conclusions













within reach














without purication






























































































Acknowledgements















References








634




Commun 2013 4 1446
















134 17396ndash17399





319


395ndash398


2008 47 58ndash77



adfm201302090


Chem Soc 2009 131 6050ndash6051


Nanoscale 2011 3

4088ndash


Sci 2013 6 1739ndash1743


Lett 2012 7 353




194006









3732


105701


470ndash482



2009 3

297ndash





2097




519


2010 22 3293ndash3297






2008 20 3516ndash3520




3668



2011 3 2280ndash2285


1152ndash1187



Soc 1991 113

1050ndash


1999 286 945ndash947




110








2006 18 789ndash794



115ndash120








3252ndash

3255


Lett 2012 3 1908ndash1915




2005 17 66ndash71




























Conclusions













within reach














without purication






























































































Acknowledgements















References








634




Commun 2013 4 1446
















134 17396ndash17399





319


395ndash398


2008 47 58ndash77



adfm201302090


Chem Soc 2009 131 6050ndash6051


Nanoscale 2011 3

4088ndash


Sci 2013 6 1739ndash1743


Lett 2012 7 353




194006









3732


105701


470ndash482



2009 3

297ndash





2097




519


2010 22 3293ndash3297






2008 20 3516ndash3520




3668



2011 3 2280ndash2285


1152ndash1187



Soc 1991 113

1050ndash


1999 286 945ndash947




110








2006 18 789ndash794



115ndash120








3252ndash

3255


Lett 2012 3 1908ndash1915




2005 17 66ndash71












































































Acknowledgements















References








634




Commun 2013 4 1446
















134 17396ndash17399





319


395ndash398


2008 47 58ndash77



adfm201302090


Chem Soc 2009 131 6050ndash6051


Nanoscale 2011 3

4088ndash


Sci 2013 6 1739ndash1743


Lett 2012 7 353




194006









3732


105701


470ndash482



2009 3

297ndash





2097




519


2010 22 3293ndash3297






2008 20 3516ndash3520




3668



2011 3 2280ndash2285


1152ndash1187



Soc 1991 113

1050ndash


1999 286 945ndash947




110








2006 18 789ndash794



115ndash120








3252ndash

3255


Lett 2012 3 1908ndash1915




2005 17 66ndash71











134 17396ndash17399





319


395ndash398


2008 47 58ndash77



adfm201302090


Chem Soc 2009 131 6050ndash6051


Nanoscale 2011 3

4088ndash


Sci 2013 6 1739ndash1743


Lett 2012 7 353




194006









3732


105701


470ndash482



2009 3

297ndash





2097




519


2010 22 3293ndash3297






2008 20 3516ndash3520




3668



2011 3 2280ndash2285


1152ndash1187



Soc 1991 113

1050ndash


1999 286 945ndash947




110








2006 18 789ndash794



115ndash120








3252ndash

3255


Lett 2012 3 1908ndash1915




2005 17 66ndash71





The Origin of High Efficiency in Low-temperature Solution

Documents

Transcript of The Origin of High Efficiency in Low-temperature Solution